1. Field of the Invention
The present invention is generally directed toward low-pressure die casting and, more particularly, toward a pressure control system and method for controlling the pressurization of low pressure die casting machines.
2. Description of Related Art
With reference to FIG. 1, a conventional die casting machine 10 is shown to include a crucible 12, a movable upper die 14, and a fixed lower die 16. The upper die 14 moves from an upper position spaced from the lower die 16 to a lower position abutting the lower die 16. Seals (not shown) fluidly seal the upper die 14 to the lower die 16. Sand-based cores 20 are received between the upper and lower dies 14, 16, and are used as the mold for forming cast parts.
The crucible 12, which receives and contains molten aluminum 22, has a pressurization input 24 by means of which pressurized air is introduced into a chamber or space 26 in the crucible 12 relatively above the molten aluminum. One or more riser tubes 28 conduct molten aluminum 22 from the crucible 12 upwardly to the dies 14, 16. Pressure developed in the space 26 forces the molten aluminum upwardly through the riser tubes 28 and around the sand cores 20. Typically, air pressure of between about 3-20 psi is used in low pressure die casting operations.
With reference to FIG. 2, a conventional control system for the die casting machine of FIG. 1 is schematically illustrated. The conventional control system includes a master programmable logic controller (master PLC) 30, a pressure controller 32, and a series of remote sensing and input/output stations or units 34.
Each of the remote units 34 provide input/output transfer and signals indicative of a sensed parameter, such as temperature and physical condition of various components (i.e., dies open/closed, valve open/closed etc.). The master programmable logic controller 30 (master PLC) holds the main program for die casting machine control. The pressure controller 32 receives, from the master PLC 30, signals generated by the remote units corresponding to various sensed parameters to start pressurization.
With reference to FIG. 3, a typical mass production recipe is illustrated. The mass production recipe program 38 includes an initial pressure ramp-up period (A), a subsequent constant-pressure period (B), and a pressure exhaust or release period (C). During the initial pressure ramp-up period, pressure within the dies increases. During the constant-pressure period (B), pressure within the dies should remain constant or static. During the pressure release period, air pressure is released from the crucible. Thereafter, the dies are opened, the cast parts are removed, and the dies are prepared for a subsequent casting cycle.
With continued reference to FIG. 3, the pressure ramp-up period includes a first portion (A') and a second portion (A"). During the first portion (A'), pressurized air is introduced into the space 26 in the crucible 12 above the molten aluminum 22 and begins to force the molten aluminum up the riser tubes 28 toward the dies 14, 16. During the second or subsequent portion (A") of the ramp-up period (A), molten aluminum is forced out of the riser tubes 28 and between the dies. The second portion (A") of the ramp-up period (A), which immediately precedes the constant pressure period (B), essentially ends when the dies 14 and 16 are full of molten aluminum.
During the constant-pressure period (B), the molten aluminum in the core 20 solidifies. Following the constant-pressure period (B), pressure is exhausted (C) from the crucible, the dies 14, 16 are opened, the formed part and cores 20 are removed from the dies, and the dies are prepared for a subsequent molding operation. The pressure ramp-up period (A) is much shorter than the constant pressure period (B). Typically, the pressure ramp-up period (A) is between about 10-20 seconds in length whereas the constant pressure period (B) is between about 200-400 seconds in length, depending upon the part being cast.
The great disparity between the relative length of the pressure ramp-up and constant pressure periods (A, B) has resulted in the prior art system not being able to numerically display or track pressure during the pressure ramp-up period (A). Accordingly, as shown in FIG. 3, the user has no numeric display of the difference between actual and desired pressure 40, 38 during the pressure ramp-up period (A). Rather, the system only shows the actual pressure 40 during the constant pressure period (B).
The aforementioned control system and method has generally worked satisfactorily in the past, but suffers from several disadvantages. Firstly, low-pressure die casting machines have numerous seals that have a tendency to leak over time. Unfortunately, the conventional system is ill equipped to compensate for such leakage. Accordingly, there tends to be wide variations in the actual pressure as compared to the desired or recipe pressure. Typically, a variation of .+-.8% between the actual pressure and desired recipe pressure occurs with the conventional system.
Also, in the conventional system, there is no means to monitor the system for gross pressure loss or lack of pressure at the beginning of the pressurization cycle (during the ramp-up period A), which would be indicative of potential catastrophic failure. As noted previously, the actual pressure 40 is not numerically displayed during the pressure ramp-up period, and no control action is taken if the actual pressure deviates significantly from the desired recipe pressure. Catastrophic failure could be the result of, for example, misalignment of the dies 14, 16, a missing sand core 20, or failure of the seals between the dies. Therefore, in the conventional system it is possible for molten aluminum 22 to be introduced into the dies 14, 16 and to leak from the dies and out of the casting machine 10, possibly causing a fire or explosion.
Finally, in the conventional system, if there is a malfunction of the pressure controller, which is proprietary, the entire die casting machine is inoperable. Such a malfunction could simply be a loss of the display unit for the pressure control system. Therefore, it is necessary to retain in inventory replacement components that are specifically dedicated to the conventional pressure control system in order to avoid or minimize costly machine downtime.
Therefore, there exists a need in the art for a pressure control system that will more accurately control the actual pressure to track the desired pressure. There also exists a need in the art for a pressure control system that will anticipate and prevent catastrophic failure. Finally, there exists a need in the art for a low pressure die casting control system that uses standard, commercially available components.